In a manufacturing process of a semiconductor integrated circuit, an exposure apparatus is used to form a pattern on a photosensitive material (to be referred to as a “resist” hereinafter) on a substrate (to be referred to as a “wafer” hereinafter). With an increase in the area of recent semiconductor integrated circuits and an advance in micropatterning, a scanning exposure apparatus called a step-and-scan exposure apparatus designed to illuminate part of a pattern on a mask as a master in the form of a slit, and to perform exposure by synchronously scanning the mask and a wafer at a constant velocity is replacing a conventional step-and-repeat exposure apparatus, a so-called stepper, which is designed for cell projection of a mask pattern.
In general, a proper exposure amount (to be referred to as a “set exposure amount” hereinafter) D (J/m2) for the formation of a proper mask pattern image is set for a resist. A scanning velocity V (mm/sec) in the scanning exposure apparatus must satisfyV≦Imax/D×Wx  (1)where Imax (W/m2) is the maximum exposure illuminance of exposure light on a wafer, and Ws (mm) is the exposure slit width on the wafer in a non-scanning direction.
According to inequality (1), the maximum scanning velocity controlled by the set exposure amount D is given byVd=Imax/D×Ws  (2)
A maximum scanning velocity Vmax determined from the performance of a stage control system, including structural/mechanical performance, is substantially determined in the scanning exposure apparatus, and the scanning velocity V must satisfyV≦Vmax  (3)
One of the factors responsible for the above requirement is that the positions of a mask and wafer cannot be properly controlled to result in a deviation (to be referred to as a “synchronization error” hereinafter), i.e., a deviation from a predetermined positional relationship between the mask and the wafer, in the scanning exposure apparatus designed to form a mask pattern on the wafer by scanning/exposing the mask and wafer while performing synchronous control to keep the positions of the mask and wafer in the predetermined positional relationship. This leads to a decrease in the resolution of a resist pattern and a deviation from the proper imaging position of the resist pattern, resulting in trouble in the manufacture of a semiconductor integrated circuit. This synchronization error is almost proportional to the scanning velocity. As the scanning velocity increases, the synchronization error increases. For this reason, the maximum scanning velocity Vmax that suppresses the synchronization error with an allowable synchronization error range is determined.
If a pulsed light source such as a KrF excimer laser or an ArF excimer laser is used as an exposure light source to meet the requirement for micropatterning, since pulsed light varies in energy for each pulse, an integrated exposure amount is made uniform within a desired precision by performing exposure with a plurality of pulsed light beams equal to or larger than a predetermined pulse count (to be referred to as a “minimum exposure pulse count” hereinafter) Pmin. For this reason, in the scanning exposure apparatus, the following inequality must be satisfied:Pmin≦Ws/V×f  (4)where f (Hz) is the oscillation frequency of the exposure light source laser.
According to the inequality, letting fmax be the maximum oscillation frequency of the exposure light source laser, the maximum scanning velocity controlled by the minimum scanning velocity controlled by the minimum exposure pulse count Pmin is given byVp=Ws/Pmin×fmax  (5)
Conventionally, as disclosed in, for example, Japanese Patent Laid-Open Nos. 10-270345 and 10-223513, a scanning velocity is determined with the oscillation frequency fmax when a low-sensitivity resist with the large set exposure amount D is to be used, or a scanning velocity is determined to set the maximum scanning velocity Vmax when a high-sensitivity resist with the small set exposure amount D is to be used, so as to satisfy inequalities (1), (3), and (4).
If the integrated exposure amount can be set to Pmin regardless of the maximum scanning velocity Vd controlled by the set exposure amount D represented by equation (2), the maximum scanning velocity Vmax controlled in accordance with the performance of the apparatus, and the value of the set exposure amount D, the minimum value of Vp controlled in accordance with the performance of the apparatus, and the value of the set exposure amount D, the minimum value of Vp controlled by the minimum exposure pulse count represented by equation (5) is determined as a scanning velocity in actual exposure operation.
An important item in manufacturing a semiconductor integrated circuit is a reduction in the size of an integrated circuit pattern. This is mainly associated with optical imaging performance and the performance of a stage control system, in particular, a in a scanning exposure apparatus.
In manufacturing a semiconductor integrated circuit, about 10 to 20 exposure processes are repeatedly performed for one wafer. As a reduction in integrated circuit pattern size, an increase in overlay accuracy of integrated circuit patterns between the respective exposure processes is associated with one of important abilities.
In the manufacturing process of a semiconductor integrated circuit, another important factor is productivity. The productivity of an exposure apparatus increases as the time required to expose one wafer shortens, i.e., the number of wafers that can be exposed per unit time (to be referred to as a “throughput” hereinafter) increases.
In a conventional scanning velocity determination method, exposure is performed at the smallest value of the maximum scanning velocity Vd controlled by the set exposure amount D, the maximum scanning velocity Vmax controlled from the performance of the apparatus, and the maximum scanning velocity Vp controlled by the minimum exposure pulse count, i.e., the highest possible scanning velocity of the maximum scanning velocities Vd, Vmax, and Vp.
This indicates that the throughput increases as the scanning velocity increases. Obviously, in a scanning exposure apparatus designed to illuminate a pattern area to be exposed in the form of a slit and to perform exposure by synchronously scanning a mask and wafer at a constant velocity, the time required to scan the pattern area to be exposed shortens as the scanning velocity increases while the length of the pattern area to be exposed in the scanning direction remains unchanged.
However, after one pattern area is exposed, both the mask stage and the wafer stage are temporarily stopped. Thereafter, the next pattern area is exposed by scanning the stages in the opposite direction. To increase the mask and wafer scanning velocities, therefore, is to prolong the time required to accelerate each scanning velocity to the above scanning velocity and the time required to decelerate each of the mask and wafer scanning velocities to 0. At a given scanning velocity or higher, the time required to scan a pattern area to be exposed shortens because of an increase in scanning velocity. However, the time required to accelerate each of the mask and wafer scanning velocities to the scanning velocity in the pattern area to be exposed and the time required to decelerate each scanning velocity to 0 is prolonged more than the time is shortened. As a result, the total time required to start driving a mask and wafer, to reach the scanning velocity in a pattern area to be exposed, and to complete a driving operation of the mask and wafer may be prolonged as the scanning velocity is increased, resulting in a reduction in throughput.